Pin pip version for now

See pypa/pip#5240

Dockerfile

@@ -3,7 +3,7 @@ FROM ubuntu:16.04
 RUN apt-get update && \
     apt-get install -y git qt5-default libraw-dev libccfits-dev wget python-pip \
     cmake pkg-config libjpeg8-dev libjasper-dev libpng12-dev libtiff5-dev && \
-    pip install --upgrade pip && pip install cpp-coveralls && \
+    pip install --upgrade pip==9.0.3 && pip install cpp-coveralls && \
     wget https://github.com/opencv/opencv/archive/3.3.0.tar.gz -O opencv-3.3.0.tar.gz && \
     tar -zxvf opencv-3.3.0.tar.gz && cd opencv-3.3.0 && mkdir build && cd build && \
     cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr .. && \